The present invention relates to a groove pattern for the friction facings of wet clutches; and more particularly to a viscous pumping groove pattern therefor.
Various groove patterns have been employed on the friction facings of clutch plates for wet clutches to move the coolant, such as oil, across the facing to remove heat and reduce the temperature gradient across the facing surface from the inner edge to the outer edge thereof.
The higher temperature in the facing of the clutch occurs at the outer edge because of the higher relative speed of the facing surface of the clutch and friction in engaging the corresponding plate of the clutch. In most groove patterns employed, the coolant is forced from the inner edge of the facing through the groove pattern to the outer edge by centrifugal forces. In some instances of operation at lower speeds, the centrifugal force is relatively low in which case the coolant does not move readily across the facing resulting in the outer edge portion being inadequately cooled. In the absence of adequate coolant, the clutch face temperature rise will lead to thermal failure of the clutch facing and may also cause decomposition of the oil coolant.
On the other hand, a higher speed rotation of the clutch plate relative to the friction surface of the centrifugal force may move the oil (coolant) through the grooves too rapidly such that the oil does not absorb the heat efficiently for cooling the clutch.
Various torque applying systems generate heat when the elements are engaging or when the clutch is slipping. In a standard automatic transmission, the elements slip relative to one another over a short period of time. Yet in other transmissions, such as CVT (continuous variable transmission), the elements slip over an extended time. Over a one-half second period of slipping of the elements of the wet clutches is an extended slip time. In the extended slip time of operation, the clutch requires better cooling to remove the build up of the heat.